Examination of the pathogenic potential of Candida albicans filamentous cells in an animal model of haematogenously disseminated candidiasis

Resveratrol-coated gold nanorods produced by green synthesis with activity against Candida albicans

Candida albicans is an opportunistic yeast capable of causing a wide range of mucosal, cutaneous, and systemic infections. However, therapeutic strategies are limited to a few antifungal agents. Inorganic nanoparticles have been investigated as carrier systems for antifungals as potential new treatments. In this study, we focused on the antifungal activity of gold nanorods, a specific rod-shaped gold nanoparticle, produced by green synthesis using resveratrol as a metal-reducing agent. The synthesis method resulted in stable control nanoparticles (AuNp) and resveratrol-coated gold nanoparticles (AuNpRSV) with medium sizes of 32.4 × 15.9 nm for AuNp, and 33.5 × 15.3 nm for AuNpRSV. Both AuNp and AuNpRSV inhibited the C. albicans grown at 2.46 µg/mL, exhibited fungicidal effects at 4.92 µg/mL, and significantly decreased filamentation, biofilm viability, reactive oxygen species production and ergosterol levels of C. albicans. In addition, exposure to AuNpRSV reduced the ability of C. albicans to grow in the presence of cell membrane stressors. Transmission electron microscopy revealed enlargement of the cell wall and retraction of the cell membrane after treatment with AuNp and AuNpRSV. Promisingly, in vivo toxicity analysis demonstrated that both nanoparticles maintained the full viability of Galleria mellonella larvae at 49.20 µg/mL. In conclusion, both gold nanoparticles exhibited antifungal activity; however, these effects were enhanced by AuNpRSV. Altogether, AuNps and AuNpRSVs are potential antifungal agents for the treatment of C. albicans infections.

Examining the effects of BRG1 over-expression on Candida albicans strains growing as pseudohyphae

The pathogen Candida albicans is pleiomorphic and grows in yeast and filamentous forms but the relationship between the regulation of different filamentous forms is unclear. BRG1 encodes a DNA binding protein which is an important regulator of morphology. Mutants lacking BRG1 grow as yeast under all conditions tested and over-expressing BRG1 drives hyphal growth even in the absence of inducing signals. A number of genetic mutants in repressors of filamentation form pseudohyphae under yeast conditions and some of these mutants can form hyphae under hypha-inducing conditions. This study examines the position of BRG1 in the regulatory networks that govern filamentation by examining the effect of over-expressing BRG1 in pseudohyphal mutants.

The emerging role of neutrophil extracellular traps in fungal infection

Fungal infections are global public health problems and can lead to substantial human morbidity and mortality. Current antifungal therapy is not satisfactory, especially for invasive, life-threatening fungal infections. Modulating the antifungal capacity of the host immune system is a feasible way to combat fungal infections. Neutrophils are key components of the innate immune system that resist fungal pathogens by releasing reticular extracellular structures called neutrophil extracellular traps (NETs). When compared with phagocytosis and oxidative burst, NETs show better capability in terms of trapping large pathogens, such as fungi. This review will summarize interactions between fungal pathogens and NETs. Molecular mechanisms of fungi-induced NETs formation and defensive strategies used by fungi are also discussed.

Rapid proliferation due to better metabolic adaptation results in full virulence of a filament-deficient Candida albicans strain

The ability of the fungal pathogen Candida albicans to undergo a yeast-to-hypha transition is believed to be a key virulence factor, as filaments mediate tissue damage. Here, we show that virulence is not necessarily reduced in filament-deficient strains, and the results depend on the infection model used. We generate a filament-deficient strain by deletion or repression of EED1 (known to be required for maintenance of hyphal growth). Consistent with previous studies, the strain is attenuated in damaging epithelial cells and macrophages in vitro and in a mouse model of intraperitoneal infection. However, in a mouse model of systemic infection, the strain is as virulent as the wild type when mice are challenged with intermediate infectious doses, and even more virulent when using low infectious doses. Retained virulence is associated with rapid yeast proliferation, likely the result of metabolic adaptation and improved fitness, leading to high organ fungal loads. Analyses of cytokine responses in vitro and in vivo, as well as systemic infections in immunosuppressed mice, suggest that differences in immunopathology contribute to some extent to retained virulence of the filament-deficient mutant. Our findings challenge the long-standing hypothesis that hyphae are essential for pathogenesis of systemic candidiasis by C. albicans.

Antifungal activity and toxicological parameters of 8-hydroxyquinoline-5-sulfonamides using alternative animal models

AIM

The purpose of this study was to evaluate the antifungal activity and toxicological parameters of 8-hydroxyquinoline derivatives PH151 and PH153 using alternative animal models, to understand their behaviour when subjected to in vivo experiments.

METHODS AND RESULTS

We used Toll-deficient Drosophila melanogaster to test the protective effect of compounds against Candida albicans infection. Toxicological parameters were investigated in chicken and zebrafish embryos. PH151 and PH153 showed low toxicity and the treated flies with these compounds had a significantly higher survival rate than untreated flies after 7 days of infection. The compounds did not cause interruption of chicken embryogenesis. Zebrafish embryos exposed to compounds showed dose-dependent toxicity.

CONCLUSIONS

The data supported the potential of PH151 and PH153 for the treatment of systemic candidiasis and demonstrated to be appropriate drug candidates for further studies using mammalian models.

SIGNIFICANCE AND IMPACT OF THE STUDY

The increased incidence of Candida infections resistant to antifungals currently available requires acceleration of the discovery of new agents with properties of inhibiting this fungal pathogen. In this study, we have described the antifungal potential and toxicity of two 8-hydroxyquinoline derivatives using in vivo alternative models, and the results confirm their potential to be developed as new drug candidates.

Mechanisms of action of antimicrobial peptides ToAP2 and NDBP-5.7 against Candida albicans planktonic and biofilm cells

Candida albicans is a major cause of human infections, ranging from relatively simple to treat skin and mucosal diseases to systemic life-threatening invasive candidiasis. Fungal infections treatment faces three major challenges: the limited number of therapeutic options, the toxicity of the available drugs, and the rise of antifungal resistance. In this study, we demonstrate the antifungal activity and mechanism of action of peptides ToAP2 and NDBP-5.7 against planktonic cells and biofilms of C. albicans. Both peptides were active against C. albicans cells; however, ToAP2 was more active and produced more pronounced effects on fungal cells. Both peptides affected C. albicans membrane permeability and produced changes in fungal cell morphology, such as deformations in the cell wall and disruption of ultracellular organization. Both peptides showed synergism with amphotericin B, while ToAP2 also presents a synergic effect with fluconazole. Besides, ToAP2 (6.25 µM.) was able to inhibit filamentation after 24 h of treatment and was active against both the early phase and mature biofilms of C. albicans. Finally, ToAP2 was protective in a Galleria mellonella model of infection. Altogether these results point to the therapeutic potential of ToAP2 and other antimicrobial peptides in the development of new therapies for C. albicans infections.

Design and synthesis of new drugs inhibitors of Candida albicans hyphae and biofilm formation by upregulating the expression of TUP1 transcription repressor gene

Candida albicans is a common human fungal pathogen that causes disease ranging from superficial to lethal infections. C. albicans grows as budding yeast which can transform into hyphae in response to various environmental or biological stimuli. Although both forms have been associated with virulence, the hyphae form is responsible for the formation of multi-drug resistance biofilm. Here, new compounds were designed to selectively inhibit C. albicans hyphae formation without affecting human cells to afford sufficient safety. The newly designed 5-[3-substitued-4-(4-substituedbenzyloxy)-benzylidene]-2-thioxo-thiazolidin-4-one derivatives, named SR, showed very specific and effective inhibition activity against C. albicans hyphae formation. SR compounds caused hyphae inhibition activity at concentrations 10-40 fold lower than the concentration required to inhibit Candida yeast and bacterial growths. The anti-hyphae inhibition activities of SR compounds were via activation of the hyphae transcription repressor gene, TUP1. Correlation studies between the expression of TUP1 gene and the activity of SR compounds confirmed that the anti-C. albicans activities of SR compounds were via inhibition of hyphae formation. The newly designed SR compounds showed 10-40% haemolytic activity on human erythrocytes when compared to 100% haemolysis by 0.1% triton employed as positive control. Furthermore, theoretical prediction of absorption, distribution, metabolism, excretion, and toxicity (ADMET) of SR compounds confirmed their safety, efficient metabolism and possible oral bioavailability. With the minimal toxicity and significant activity of the newly-designed SR compounds, a future optimization of pharmaceutical formulation may develop a promising inhibitor of hyphal formation not only for C. albicans but also for other TUP1- dependent dimorphic fungal infections.

Candida spp. and phagocytosis: multiple evasion mechanisms

Invasive fungal infections are a global health problem, mainly in hospitals, where year by year hundreds of patients die because of these infections. Commensal yeasts may become pathogenic to human beings, affecting mainly immunocompromised patients. During infectious processes, the immune system uses phagocytes to eliminate invader microorganisms. In order to prevent or neutralize phagocyte attacks, pathogenic yeasts can use virulence factors to survive, as well as to colonize and infect the host. In this review, we describe how Candida spp., mainly Candida albicans, interact with phagocytes and use several factors that contribute to immune evasion. Polymorphism, biofilm formation, gene expression and enzyme production mediate distinct functions such as adhesion, invasion, oxidative stress response, proteolysis and escape from phagocytes. Fungal and human cells have similar structures and mechanisms that decrease the number of potential targets for antifungal drugs. Therefore, research on host-pathogen interaction may aid in the discovery of new targets and in the development of new drugs or treatments for these diseases and thus to save lives.

The planarian Schmidtea mediterranea is a new model to study host-pathogen interactions during fungal infections

Candida albicans is one of the most common fungal pathogens of humans. Currently, there are limitations in the evaluation of C. albicans infection in existing animal models, especially in terms of understanding the influence of specific infectious stages of the fungal pathogen on the host. We show that C. albicans infects, grows and invades tissues in the planarian flatworm Schmidtea mediterranea, and that the planarian responds to infection by activating components of the host innate immune system to clear and repair host tissues. We study different stages of C. albicans infection and demonstrate that planarian stem cells increase division in response to fungal infection, a process that is likely evolutionarily conserved in metazoans. Our results implicate MORN2 and TAK1/p38 signaling pathways as possible mediators of the host innate immune response to fungal infection. We propose the use of planarians as a model system to investigate host-pathogen interactions during fungal infections.

Overview of selected virulence attributes in Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Trichophyton rubrum, and Exophiala dermatitidis

The incidence of fungal diseases has been increasing since 1980, and is associated with excessive morbidity and mortality, particularly among immunosuppressed patients. Of the known 625 pathogenic fungal species, infections caused by the genera Aspergillus, Candida, Cryptococcus, and Trichophyton are responsible for more than 300 million estimated episodes of acute or chronic infections worldwide. In addition, a rather neglected group of opportunistic fungi known as black yeasts and their filamentous relatives cause a wide variety of recalcitrant infections in both immunocompetent and immunosuppressed hosts. This article provides an overview of selected virulence factors that are known to suppress host immunity and enhance the infectivity of these fungi.

Candida albicans Pathogenesis: Fitting within the Host-Microbe Damage Response Framework

Historically, the nature and extent of host damage by a microbe were considered highly dependent on virulence attributes of the microbe. However, it has become clear that disease is a complex outcome which can arise because of pathogen-mediated damage, host-mediated damage, or both, with active participation from the host microbiota. This awareness led to the formulation of the damage response framework (DRF), a revolutionary concept that defined microbial virulence as a function of host immunity. The DRF outlines six classifications of host damage outcomes based on the microbe and the strength of the immune response. In this review, we revisit this concept from the perspective of Candida albicans, a microbial pathogen uniquely adapted to its human host. This fungus commonly colonizes various anatomical sites without causing notable damage. However, depending on environmental conditions, a diverse array of diseases may occur, ranging from mucosal to invasive systemic infections resulting in microbe-mediated and/or host-mediated damage. Remarkably, C. albicans infections can fit into all six DRF classifications, depending on the anatomical site and associated host immune response. Here, we highlight some of these diverse and site-specific diseases and how they fit the DRF classifications, and we describe the animal models available to uncover pathogenic mechanisms and related host immune responses.